1. Field of the Invention
The present invention relates to a measurement method using a sensor. The sensor is typically a bulk acoustic wave resonator (BAWR), either a thin film bulk acoustic resonator (FBAR) or a solidly mounted resonator (SMR) device. The invention also relates to a sensor system incorporating such a sensor and to the sensor itself. The present invention has particular applicability, but not exclusive applicability, to the use of such a sensor in gravimetric-based sensing.
2. Related Art
EP-A-950282 [Reference 14] discloses a micromechanical resonant sensor in the form of a microbeam for use in various applications such as pressure sensing, strain sensing or as an accelerometer. The problem addressed in EP-A-950282 is that the resonant frequency of a sensor formed of a single material shifts with temperature, due to temperature-induced changes in density, elastic moduli and dimensions. EP-A-950282 addresses this problem by incorporating a material with a different coefficient of thermal expansion in order that the resultant composite microbeam has a near-zero resonant frequency shift with variations in temperature.
Pang et al (2008) [Reference 15] discloses a film bulk acoustic wave resonator (FBAR) formed using aluminium nitride and a temperature compensation material (SiO2). The first order temperature coefficient of stiffness of AlN and Mo (the electrode material) is negative, whereas SiO2 has a positive temperature coefficient of elastic modulus at around room temperature. By introducing a SiO2 film of an appropriate thickness, the temperature dependence of the resonant frequency of the resonator can be reduced.
Garcia-Gancedo et al (2010) [Reference 16] discloses the formation of high quality ZnO films whose resonant quality is tested by generating bulk acoustic waves (BAW) in the film. FBARs were manufactured by depositing ZnO films on polished Si single crystal substrates.
It is known that FBARs and SMRs are of interest, for example, in the manufacture of gravimetric biosensors. The selective attachment or adsorption of species at the surface of the sensor reduces the resonant frequency of the device. FBARs and SMRs are of particular interest for this application due to the potential for these devices to have high quality factor (Q), to allow relatively small shifts in resonant frequency to be detected, and as such are considered to be potentially more sensitive than quartz crystal microbalances (QCM).
For example, Garcia-Gancedo et al (2011) [Reference 17] reports details of the use of a ZnO-based FBAR device as a gravimetric biosensor. Garcia-Gancedo et al (2011) points out that typical QCM devices have resonant frequencies in the range 5-20 MHz, whereas the ZnO-based FBAR devices can be formed with resonant frequencies in the range of 1.5 GHz. This significant rise in resonant frequency provides the key to increased sensitivity.
Nakamura et al (1981) [Reference 18] discloses a piezoelectric resonator of a ZnO/SiO2-diaphragm supported on a silicon wafer. Nakamura et al (1981) describe a “temperature coefficient of frequency” in ppm/° C., i.e. the variation in the resonant frequency with temperature. The aim of Nakamura et al (1981) is to provide a structure which has as low a temperature coefficient of frequency as practicable. Since ZnO and SiO2 respectively have positive and negative temperature coefficients of frequency, the disclosure in that document is to select a suitable thickness for the ZnO and SiO2 films in order to try to provide a zero temperature coefficient of frequency for the composite structure. For the fundamental mode, it is found that the ideal ratio of the thickness of the ZnO and SiO2 films is 2. The ZnO and SiO2 films are formed on the silicon wafer with upper and lower electrodes sandwiching the ZnO film. The silicon wafer is etched to form the ZnO and SiO2 films into a composite diaphragm structure. The actual device reported in Nakamura et al (1981) has a temperature coefficient of frequency of 10 ppm/° C. at 25° C.